Why was my nav signal split this way?

[bradp]

Figuring out wiring runs for a future upgrade and came across my diplexer (triplexer) on the avionics shelf.  I’m thinking it was done to avoid multiple wiring runs from a single antenna at the top of the tail   

The first split (that feeds the input of the triplexer) passes through to the panel where it encounters another diplexer and splits to Nav and GS for my 430W.  The GS output of this box goes to a KN-75 and the nav output goes to the KX-170b.  

I’m going to likely remove the KX-170 and and Kn-75, and install a new nav com.  Seems like the smart move would be to use a single run of RG-400 to the cockpit and put a quad mounted near the firewall.  This should minimize signal degradation as much as possible    

https://www.aircraftspruce.com/catalog/avpages/ramiav585.php

[Jesse Saint]

That all depends on how your new Nav radio splits the VLOC and G/S frequencies. Many modern Nav/Coms diplex internally, in which case you would need a signal splitter to send the full signal to both radios, then a diplexer to separate them for the 430. The unit you picture is, in my experience, the most common because many planes have 2 Nav radios, but only 1 with glideslope. I have never seen it done as your picture shows. That looks like an old computer network splitter before the Cat5 network days. 

[EricJ]

That sounds reasonable.   Your existing system is a little sketchy the way it was teed at the input to the first splitter (shown).   New cable and a new splitter should improve things a bit.

 

[Yetti]

1 hour ago, Jesse Saint said:

 That looks like an old computer network splitter before the Cat5 network days. 

You are thinking Token Ring

[Yetti]

7 hours ago, Yetti said:

You are thinking Token Ring

or Ten based T    I forget.

[tmo]

Or 10BASE5 or 10BASE2

[bradp]

I remember token ring.  It’s like today’s can bus ;-). 

@Jesse Saint Jesse the new box is going to be a 255.  Manual references a TSO 34() splitter for Nav/Loc/GS inputs instead of a diplexer/triplexer.  Looks like the insertion loss for a CI-1125 is 3.something dB and a CI-5120 (pricey btw) is 1 dB.  I might try to find a part number on the token ring splitter I already have in the pic, and move that to the firewall,  connect it to the diplexer for the GNS and to the GNCs single input. 

Heres the single best explanation of bandpass filtering and signal loss of the various types of splitters and diplexer I’ve read (from Vans)

 

Quote:

With a dedicated antenna for each input, practically all received input power from the antenna (minus line losses, etc.) will be available to each receiver, giving the best range of reception. A VOR/LOC (or NAV) antenna is optimized for the VOR/LOC frequency range of 108-118 Mhz, but does a fair job of also picking up the glideslope frequency range of 329-335 Mhz (~third harmonic of 108-118 Mhz). Ideally, you would have a separate GS antenna optimized for that frequency band, but that is a more expensive option. Since the GS signal is so localized to the approach end of the airport and range is not a huge issue, a nav antenna does an acceptable job of picking it up.

Any use of splitters, diplexers, or triplexers is a compromise. All of these devices provide impedance matching of outputs to inputs, minimizing impedance mismatch (reflection) losses.

What is often referred to as a "splitter" (ex: Comant CI-5120 or Mini-Circuits ZFSC-2-1) splits the entire received signal to two separate outputs, each output including both the VOR/LOC and GS components and about half the power or a little less (-3 to -4 dB loss) of the received signal. This will reduce the range of the received signal versus a dedicated antenna.

A "diplexer" (ex: Comant CI-507 or RAMI AV-570) is used to break out the separate frequency ranges to two separate outputs (one for VOR/LOC and one for GS) by filtering out each component (with usually less than -1 dB loss for each). In other words the GS signal on the GS output of a diplexer will only suffer about -1dB loss of the input GS component, but the VOR/LOC signal on the GS output with be filtered out by about -30 dB or more, and vice-versa for the VOR/LOC output of the splitter. The SL-30 has a built-in "diplexer", the GNS-430 does not.

A "triplexer" (ex: Comant CI-505 or RAMI AV-571) is a combination of the two. It has one input connector from the antenna, two VOR/LOC outputs and one GS output. I'm not sure if the signals are "diplexed" first to the GS component and VOR/LOC component, and then the VOR/LOC further "split" to two outputs, or if it is split first and then diplexed. There is a difference. If it is diplexed first (which I suspect it is), the GS component of each of the VOR/LOC outputs would be at a very low level (-30 dB) compared to the input, but the GS component at the GS output would only be down about -1 dB). If it is split first (which I doubt) then the output at the GS output would be down about -4 dB as would each of the VOR/LOC outputs (but each of the VOR/LOC outputs would contain a GS component). If somebody knows more about how the "triplexer" actualy splits the signals, please pipe in.

So, the best compromise in my mind when connecting a single nav antenna to a GNS-430/530 and SL-30, would be to connect the nav antenna to the input of a "splitter" (I used the Mini-Circuits ZFSC-2-1) with one of the outputs connected to the SL-30 (with its built in "diplexer") and the other output connected to the input of a "diplexer" (I used the RAMI AV-570) and the two outputs of the diplexer ("VOR/LOC" and "GS") connected to the corresponding antenna inputs of the GNS-430/530. I believe this is what Stein and others have recommended as well. But hopefully now you know why

[EricJ]

That's a pretty good functional explanation.

Reducing insertion loss is always a Good Thing, but, yeah, may come at a price.

The technology is going the right way so that the receiver can split things up and process it as needed, i.e., just send the combined VOR/LOC and GS signals together into a single port and do the signal processing and tuning inside the radio.   These days all of it can be done digitally, which means minimal distortion and SNR loss along the way.    Putting the signal through multiple, passive, analog splitters and diplexing filters before it ever gets to the radio helped in the old days when radios had limited dynamic range and selectivity, but these days it just adds distortion and reduces SNR. 

But if you're trying to mix new and old equipment you still get to do the diplexer/splitter dance if something (e.g., your 430) requires the VOR/LOC and GS to be separated.  It sounds like you're on the right track,.   New RG-400 wherever you can manage it won't hurt.

[bradp]

I have an OMG my wallet hurts number of feet of RG-400 on the way and will replace wherever I can.  

[Aerodon]

Brad,

I can tell you first hand that you will have much improved Nav performance just by using the correct splitters and combiners.  Sometimes its easy - One Comant splitter can provide separate outputs for each GS and each VOR. (1 in 4 out).  Typical for GNS430/530 combinations.

Radios like the GNS480 and SL 30 need a combined Nav / GS signal. I use the Mini-Circuits ZFSC-2-1 to split the Nav antenna signal to nav 1 and nav 2.    These have a wide frequency range necessary to split the signal for both VOR and GS frequencies.

And them sometimes you have a combination of the above.  You may have to split the Nav 2 signal into GS/VOR, use a Comant splitter with the exact descriptions.  Don't try drive a GS receiver from a VOR 'output' from the splitter and vice versa.

Don't leave any ports open, and don't use t connections as shown in you picture.

You will get remarkably better performance, even with your old cables.

 

Aerodon

 

 

 

 

 

 

[bradp]

@Aerodon Don I saw the mini circuits unit and although not “tso’d“ looks like it’s the best solution. Also it’s OEM for a number of aircraft including old Cessna 337s from what I can gather.  I’ll dump the splitter and go with the 2-1+.  Thanks !